The incidence of diabetes mellitus in Sweden in the 15-34 year age group was prospectively studied on a nationwide basis, beginning 1 January 1983. A total of 1,214 male and 720 female cases of newly-diagnosed (excluding gestational) diabetes were reported over a 5-year period. This corresponds to an incidence of 20.5 per 100,000/year in male subjects and 12.7 per 100,000/year in female subjects. Most cases were classified as Type 1 (insulin-dependent) diabetes, with an incidence of 15.9 in males and 8.6 in females. The incidence of Type 1 diabetes decreased gradually with age, while the incidence of Type 2 (non-insulin-dependent) diabetes increased. A male predominance was found in all age groups, with a male-to-female ratio of 1.8:1 for Type 1 diabetes and 1.3:1 for Type 2 diabetes. Maximum blood glucose concentration at diagnosis was significantly higher in males than in females in both Type 1 and Type 2 diabetic subjects. In contrast, the percent desirable weight was significantly higher in females, both in Type 1 and Type 2 diabetic subjects. The difference in diabetes incidence therefore cannot be attributed to any methodological error. The present finding of a marked male predominance after puberty in Type 1 diabetes in an ethnically quite homogeneous population supports the hypothesis that environmental risk factors and life-style are important for the development of the disease.